In the aerospace industry, honeycomb core is commonly employed in the fabrication of various panels and assemblies to provide strength and light weight. The honeycomb core is bonded between face sheets which close the transverse cells defining the core.
Engine nacelle panels can be modified to serve as Helmholtz resonators for attenuating engine noise. This involves perforating one or both of the face sheets such that the cells of the honeycomb core communicate with the outside via small holes. The cell and hole sizes as well as the thickness of the face sheet are selected such that the structure is tuned to resonate at a preselected engine frequency. The cells of the honeycomb core act as multiple resonant chambers wherein the sound waves are broken down into waves of different, and usually higher, frequencies and wherein some of the acoustic energy is transformed into heat which dissipates into the atmosphere. U.S. Pat. Nos. 3,948,346 and 3,910,374 illustrate examples of such acoustic liners.
In some applications, it is desirable to install one or more septums in the honeycomb core to divide each cell into a plurality of cell segments such that resonance occurs at two or more frequencies. However, the typical method of installing such a divider involves splitting the honeycomb core into two layers and bonding a sheet of perforated material between the two layers. This approach is time consuming and adds expense to the resultant sound attenuation panel. In addition, contoured honeycomb cores formed according to this technique require specially formed sheets of divider material.
A need thus exists for an improved method for installing a septum in honeycomb core which does not require splitting the core.